SummaryThis study addresses relationships between leaf phenology, xylogenesis, and functional xylem anatomy in two ring-porous oak species, the temperate Quercus robur and the subMediterranean Q. pyrenaica.Earlywood vessel (EV) formation and leaf phenology were monitored in 2012 and 2013. Ten individuals per species were sampled at each of three sites located in NW Iberian Peninsula. EV areas measured on microcore sections were used to calculate the hydraulic tree diameter (D h ), in order to model relationships to phenology. Thermal requirements were evaluated using growing degree days (GDD).A species-specific timing of growth resumption was found. The onset of EV formation and budburst were associated to a particular GDD in each species. The onset and duration of EV enlargement affected D h (and EV size) in Q. robur, but hardly in Q. pyrenaica.The relationship between the timings of EV formation and xylem structure appears to be stronger for the temperate oak, whose larger vessels may result from thermal-induced earlier resumption. In contrast, the sub-Mediterranean oak would maintain a more conservative hydraulic architecture under warming conditions.

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Summary Environmental conditions and the structure of the dormant cambium are assumed to affect seasonal patterns of cambial activity, hence controlling allocation of non‐structural carbohydrates (NSC) to growth. However, seasonal dynamics of xylogenesis, and their connections with NSC content and dormant cambium size, have been rarely assessed along an environmental gradient. We monitored xylogenesis and leaf phenology during 2012 and 2013, and NSC in 2012, for the drought‐sensitive Quercus robur and the drought‐tolerant Quercus pyrenaica along a water‐availability gradient in the north‐western Iberian Peninsula, and analysed dependencies of xylem production and phenology on the number of cells in dormant cambium. Study oak species showed comparable seasonal fluctuations in cambial activity and NSC content, despite Q. pyrenaica had a shorter growing season and a lower wood production than Q. robur. A sharp drop in spring NSC levels at all study sites evidenced that stored carbohydrates were crucial for earlywood formation. Under drier conditions, both species extended the growing period in spring and autumn, but reduced and even stopped xylogenesis in summer, showing an enhanced NSC accumulation before dormancy. A higher number of cells in dormant cambium of large dominant oaks accounted for their wider xylem increments and longer active periods. Our study demonstrates that xylogenesis is modulated by predisposing effects of dormant cambium size on xylem production and growing season length. Moreover, the high plasticity of cambial activity in deciduous oaks would confer resistance against recurrent summer drought through the improvement of the NSC status. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12789/suppinfo is available for this article.

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We analysed climate–growth relationships for two ring-porous oak species (Quercus robur L. and Q. pyrenaica Willd.) at one site in northwestern Spain. Increment cores from 12 living trees per species were taken for ring and earlywood vessel measurements, considering three width variables (for earlywood, latewood, and the whole ring) and six anatomical variables (based on vessel area distribution, number of vessels, and conductivity) in the earlywood. We used standard dendrochronological procedures and compared each mean series with meteorological records of temperature, precipitation, and estimates of soil water content. Earlywood width was negatively related to precipitation in the previous late season for Q. robur, but it was controlled by the previous spring temperature for Q. pyrenaica. These responses were highly dependent on vessel number and probably related to carbohydrate storage dynamics but differed between species. Vessel size showed a stronger relationship to climate and was mainly coupled to water excess during the winter for Q. robur but dependent on quiescence temperature for Q. pyrenaica; also, climatic signal in vessel size was maximized by the largest vessels for Q. pyrenaica but only increased slightly for Q. robur. Consequently, vessel parameters showed a much stronger climate signal than ring width and were successful at highlighting the differences between both oak species.